Numerous microwave communications and sensing devices require an antenna for signal transmission and reception. At microwave frequencies of one GHz or more, multilayer microstrip antennas are commonly employed. These antennas may be single or multi-patch antennas which provide energy transmission or reception in many directions simultaneously. To focus the energy, an array of patch antennas fed by a common transmission line is often employed.
These antennas are commonly constructed from sandwiched, parallel laminated layers of insulating substrates and conducting metal sheets. Each metal layer incorporates a two-dimensional pattern designed to efficiently channel radio frequency energy. The design is accomplished by first employing analysis rules and subsequently improving and simulating the design according to procedures known to practitioners of the art of antenna design. Given a set of antenna parameters such as operation frequency, bandwidth, directivity gain, and input impedance, and selected material properties and desired attributes such as dielectric constant, loss factor, sandwich layer thickness and minimal feature size, the best performing antenna is designed.
Typical current practice in microwave antenna fabrication employs a modified form of printed circuit board (PCB) manufacturing technology, in which layers of copper foil are etched according to a design pattern and are sequentially laminated between and/or onto layers of a low-loss dielectric substrate material such as PTFE, or a composite material, e.g., a resin-impregnated reinforcing fiber mat.
The current wide increase in the use of wireless technology for a host of personal and commercial applications has created a need for small microwave antennas that can be incorporated into clothing, vehicles, briefcases, and the like. If incorporated into ordinarily flexible fabric articles, such as wearable clothing, tote bags, or vehicle covers, a rigid PCB antenna would create undesirable rigid portions, tending to form objectionable lumps that would be uncomfortable in clothing, and would cause increased fabric wear, reducing article lifetimes and limiting applications. Rigid PCB antennas are also unsightly without added encumbering packaging and are considered unacceptable for many applications, indoor and outdoor. With the increasingly ubiquitous wireless applications, improved aesthetic qualities are desirable. Materials of an order of magnitude less cost are also desired to enable wider applications.
Other problems inherent in the use of PCB antennas are as follows. Transitioning a conventional PCB antenna design to manufacturing can require several weeks; a more rapid turn around time enabling custom applications at a reasonable cost is desirable. A conventional PCB antenna can deform under heat and can transmit excessive levels of acoustic noise and vibration. Further, conventional PCB techniques employ environmentally hostile etching or time-consuming milling steps to implement the desired waveguide patterns into the metal foil.
There are applications for the incorporation of antennas into airframes, ship superstructures or composite support beams for buildings for which conventional PCB techniques are not suitable, due to structural weakening that occurs when a conventional antenna laminate is incorporated into a composite superstructure; due to incompatibility of the materials used, the antenna might tend to delamination. An antenna construction technology that enables an integral antenna to be made without reducing the electronic characteristics or working life of the antenna is needed.
Conventional arrays of antennas are limited in size due to the manufacturers' ability to make and work with large sheets of PCBs. Therefore, arrays covering hundreds of square meters, such as those desired for satellite applications, are very difficult to manufacture. Inevitably, they would have to be fabricated in panels, further complicating the structural and connection issues.
Various paint-on or print-on techniques employing conductive paints or inks have been suggested as alternatives to the conventional PCB laminate methods. These applique methods produce antennas that crack and flake when the antenna is flexed, or if the underlying substrate expands and contracts due to thermal variation, resulting in degraded performance. Flexing due to predeployment packaging as well as vibration can produce differential stress on these antennas and contribute to antenna failure. Additionally, exposure to ultraviolet light and to atmospheric oxygen causes erosion of the metallic applique that greatly reduces performance and lifetime.